1. Field of the Invention
The present invention relates to an optical disc capable of printing on the label surface by an ink jet printing process such as compact disc (CD) and digital video disc (DVD) and a process for the production of a label surface therefor.
2. Description of the Related Art
Heretofore, in a machine for mass-producing optical discs such as read-only CD and DVD, it has been usual that a label having the description of title of data stored in the discs, name of manufacturer, name of distributor, logo mark and other images is printed on the surface of the discs using a dedicated screen printing machine or offset printing machine.
FIG. 2 illustrates a flow chart of a step of producing DVD. In the following description, where the same constituents are used, the same reference numerals are used even in different drawings. In FIG. 2, a laminating disc 9 produced by injection molding using a forming device 8 is subjected to vacuum metallizing with aluminum on the surface thereof in a vacuum evaporating device 10 to form a laser beam reflective layer. Two sheets of disc 9 having two different data layers are then laminated with each other by spin coat adhesion or like method in a laminating device 11. The laminate thus obtained is then ultraviolet-cured. A base layer ink is screen-printed or otherwise spread over the laminate on the side thereof opposite the reflective surface, and then ultraviolet-cured using a base layer printing device 12. Using a label printing device 13, a label is color-printed on the laminate by screen printing process or offset printing process, and then ultraviolet-cured to produce DVD 14. However, since printing by these printing machines requires much time and expenses to produce printing plate and adjust colors, ink jet printers are used to record images in the case of small scale production of media such as printable CD-R and DVD-R and rewritable CD-RW and DVD-RW. In the case where ink jet printers are used to record images, it is necessary that an ink-receiving layer have been previously formed on the surface of the optical disc because the surface of the optical disc such as CD and DVD has no ink absorptivity. An ink is ejected onto the ink-receiving layer through an ink jet head to form an image thereon. Thus, an optical disc allowing printing by an ink jet printer is disclosed. (See, e.g., JP-A-8-102098 (FIG. 3, page 2), JP-A-10-188345 (FIG. 3, page 2))
FIG. 3 is a sectional view of an optical disc adapted for ink jet printing. In FIG. 3, the reference numeral 1 indicates an optical disc, the reference numeral 2 indicates an optical disc substrate having a data recording surface formed thereon, the reference numeral 3 indicates a clamp hole for setting the optical disc on a drive or player, the reference numeral 4 indicates a white base layer, the reference numeral 5 indicates an ink-receiving layer, and the reference numeral 6 indicates a label image printed on the ink-receiving layer by an inkjet printer. The optical disc substrate 2 is normally in the form of disc having an outer diameter of 120 mm, an inner diameter, i.e., clamp hole 3 diameter of 15 mm and a thickness of 1.2 mm and comprises a transparent plastic such as polycarbonate as a base material. The white base layer 4 is formed when it is desired to block the color of the recording layer or metal deposit in the optical disc substrate in order to make it possible to perform natural color printing. The white base layer 4 is obtained by spreading a white ultraviolet-curing ink over the substrate by a screen printing process or the like, and then curing the ink. The ink-receiving layer 5 is adapted to absorb and retain the ink jet recording ink, allowing image formation, and is normally formed by spreading an ink-like composition comprising a hydrophilic resin, a photo-polymerizable monomer, a photopolymerization initiator, a filler and other additives incorporated therein in admixture by a screen printing process or the like, and the curing the coat. In this arrangement, printing using an ink jet printer can be conducted.
However, since the optical disc having the aforementioned arrangement has moisture absorption in the ink-receiving layer and an insufficient ink fixability, the resulting image cannot be sometimes provided with a sufficient water resistance. Further, since the ink-receiving layer has a low strength, the resulting image is subject to exfoliation or damage, occasionally making it impossible to provide the label-printed surface with a sufficient durability. In order to solve these problems, a method for the production of a label for disc has been disclosed which comprises forming an overcoat layer (protective layer) of ultraviolet-curing resin on the printing surface to enhance the durability of the label-printed surface. (See, e.g., JP-A-9-330535 (FIG. 4, page 3)). FIG. 4 is a sectional view of an optical disc adapted for ink jet printing having a related art protective layer. In FIG. 4, the reference numeral 1 indicates an optical disc, the reference numeral 2 indicates an optical disc substrate having a data recording surface formed thereon, the reference numeral 3 indicates a clamp hole for setting the optical disc on a drive or player, the reference numeral 4 indicates a white base layer, the reference numeral 5 indicates an ink-receiving layer, the reference numeral 6 indicates a label image printed on the ink-receiving layer by an ink jet printer, and the reference numeral 15 indicates a protective layer formed by an ultraviolet-curing resin.
The protection of the ink jet-receiving layer and the printed image by such an arrangement makes it possible to prevent the damage or exfoliation of the printing surface.
In the aforementioned method for forming a protective layer for printing surface by an ultraviolet-curing resin, it is necessary that the ultraviolet-curing resin spread over the surface of the disc substrate on which a label image has been formed be irradiated on the surface thereof with a high intensity ultraviolet ray to cure. Color ink jet recording inks preferably comprise organic dyes or pigments incorporated therein as colorants. In particular, organic dyes normally exhibit a higher color developability and saturation and a better adaptability to ink-receiving layer and thus can form a higher quality image than organic pigments. However, these organic coloring matters, particularly organic dyes, can be easily affected by ultraviolet rays and most of them undergo molecular modification and discoloration when irradiated with ultraviolet rays. Accordingly, the related art technique for forming protective layer is disadvantageous in that when irradiated with ultraviolet rays for curing the ultraviolet-curing resin spread over the ink jet-printed surface, the organic coloring matters in the ink jet-printed surface undergo modification and discoloration that cause image deterioration.